Process for protecting titanium and titanium alloys against corrosion by oxidizing acid media



United States Patent 3,457,103 PROCESS FOR PROTECTING TITANIUM AND TI-TANIUM ALLOYS AGAINST CORROSION BY OXIDIZING ACID MEDIA Hans Keller,Frankfurt am Main, Karl Risch, Hoiheim,

Taunus, and Winfried Altheu, Kelkheim, Taunus, Germany, assignors toFarbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning,Frankfurt am Main, Germany, a corporation of Germany No Drawing.Continuation-impart of application Ser. N 0. 319,567, Oct. 28, 1963.This application Apr. 4, 1967, Ser. No. 628,268 Claims priority,application Germany, Dec. 7, 1962, F 38,483 Int. Cl. C23c 11/08; B44111/36 U.S. Cl. 117135.1 3 Claims ABSTRACT OF THE DISCLOSURE Oxidizingacid media, especially nitric acid, aqua regia and chromosulfuric acid,do not, or only slightly attack titanium and titanium alloys even attemperatures exceeding 100 C., when acting in the presence of silicon orsiliceous substances. The anticorrosive effect is observed inpractically all siliceous substances, even in those known to be inerttowards oxidizing acid media. The corrosion attack in the vaporous phaseis particularly avoided by vaporizable or sprayable siliceous compounds.Especially suitable are elementary silicon, iron silicide, precipitatedsilicic acid, quartzand glass powder, kaolin, asbestos, and among thevolatile or sprayable siliceous compounds the silicon halides, thehalogeno-silanes and, in particular, silicone oils.

The present application is a continuation in part of the copendingapplication Ser. No. 319,567, now abandoned.

The present invention provides a process for protecting titanium andtitanium alloys against the corrosion caused by oxidizing acid media attemperatures exceeding 100 C. By oxidizing acid media there are, inparticular, understood nitric acid and mixtures of nitric acid andhydrochloric acid, so-called aqua regia, moreover the mixtures ofconcentrated sulfuric acid and chromic acid, known as chromosulfuricacid.

Oxidizing acid media of this type vigorously attack titanium andtitanium alloys which may especially contain aluminum, vanadium,molybdenum and palladium, at elevated temperatures, in particular attemperatures exceeding 100 C.

It has now been found that the corrosion of titanium and titanium alloyscaused by oxidizing acid media can be noticeably reduced or completelyavoided even at temperatures exceeding 100 C. by allowing the oxidizingacid media to act upon the titainum or the titanium alloy in thepresence of silicon or a siliceous compound.

This observation is surprising since most of the compounds of siliconare practically inert towards oxidizing acid media and since siliceouscompounds which are not inert under such conditions are converted intopractically inert compounds by hydrolysis.

It now appeared that protection is obtained when the oxidizing acidmedium contains, for example, quartz sand or glass powder. Theprotection is even more effective in the presence of siliceous compoundswhich are less inert. Compounds of this type are, in particular,precipitated silicic acid, iron silicide, alkali metal silicates. kaolinand asbestos powder. It is a matter of course that elementary silicon isalso effective.

With the use of solid and non-volatile siliceous substances it is,naturally, only possible to protect the part of the metal surface whichcomes into contact with the "ice liquid phase of the oxidizing acidmedium. The parts of the apparatus which come into contact with thevapor of the oxidizing acid medium or in which the vaporous phase iscondensed, can, however, be protected by volatile or sprayable siliceoussubstances. Members of this group of substances are, for example thesilicon halides such as SiCl the halogeno-silanes such as SiHCl thesiloxanes such as (SiH O and, in particular, the silicone oils.

Silicone oils are particularly suitable for carrying out the process ofthe invention since they are easy to handl-e and to dose and since theirhandling involves no hazards.

In apparatuses which are in contact partly with the liquid phase andpartly with the vaporous phase of an oxidizing acid medium, it may beadvantageous to use various siliceous substances for the protectionagainst corrosion. It is, for example, possible in the case ofdistilling columns, to introduce kaolin into the sump and to spraysilicone oil into the vaporous phase.

The amount of siliceous substance is not critical. The spaces to beprotected should, however, always contain such an amount of siliceoussubstance that, per square meter of the surface to be protected, 10milligrams of silicon are present, when siliceous compounds are used, itshould be an amount equivalent to that of the silicon. When compoundsare used which are volatile with the vapor of the oxidizing acid medium,the silicon level may be maintained by a continuous or portionwise feed.In many cases, non volatile siliceous substances can be added to theoxidizing acid medium in an excess sufiicient to do without a feedingover prolonged intervals.

In the following example and tables there are described a number ofcorrosion tests for illustrating further particulars of the process ofthe invention and of the protection of titanium and the alloys thereofagainst oxidizing acid media. However, the examples are not intended tolimit the invention thereto.

Example For the corrosion tests tubes of pure titanium were used whichserved as testing vessels and as corrosion samples simultaneously. Theyhad an inside diameter of 20 mm., an inner height of mm. and an innersurface of 97 cm. One end of the tubes was closed by welding with a puretitanium plate. On the other end the tube was closed so at to begastight by means of a metal cover and a sealing plate ofpolytetrafiuoroethylene with the use of a clamping device. The titaniumtubes were filled to about /5 of their height with the corrosive medium,placed in thick-walled steel containers and heated at the requiredtemperature in a furnace with circulating air. The temperature wasmaintained constant by means of a regulator. In the following Tables 1,2, and 3 are indicated the test results obtained, i.e. the anticorrosiveeffect on pure titanium against nitric acid and mixtues of nitric acidand hydrochloric acid.

In Table 1 are recited the corrosion values obtained in the treatment ofpure titanium with nitric acid of varying concentrations and atdifferent temperatures.

In Tests 1 to 7 severe care was taken that the results were notinfluenced by siliceous substances. That is the reason why no glassvessels were used. As a measurement for the corrosion there is indicatedin the last column of Table 1 the linear corrosion velocity inmillimeters per annum. In Tests Nos. 1, 2 and 7 the nitric acid formedon the pure titanium a firmly adhering layer of corrosion products whichcould not be removed quantitatively without carrying off unattackedtitanium. Therefore, reliable results cannot be given.

Table 2 contains the experimental data with respect 3 to the stabilityof pure titanium towards HNO with different additions under varyingconditions.

In column 7 of Table 2, V stands for vaporous phase and L for liquidphase. The plus means that no corrosion could be observed. The minusindicates that the nitric acid condensed in the vaporous phase didattack the metal. The corrosion velocity in each case was about 2millimeter per annum, corresponding to the value indicated in Table 1for an about 30% nitric acid at 100 C. to indicates that the protectionwas not uniform, some spots of the metal surface having been attacked.

Table 3 indicates the linear corrosion velocity of titanium under theaction of mixtures of nitric acid and hydrochloric acid of varyingconcentrations and at different temperatures and illustrates theanticorrosive effect obtained by carrying out the process according tothe invention.

the same concentration and temperature of the acid, a corrosion did nottake place when a very small amount of silicone oil was added to thenitric acid. Neither on the parts in contact with the vaporous phase noron the parts in contact with the liquid phase of the nitric acid acorrosion could be observed. With the additions used in Tests Nos. 816only the parts in contact with the liquid phase of the acid wereprotected against corrosion.

The weaker anticorrosive effect obtained in Tests Nos. 11 and 14 ismainly due to the fact that glass powder and quartz powder are attackedto a very small extent only by nitric acid so that the protectiondevelops only slowly.

The tests have further revealed that in the liquid phase the protectivelayer on the metal was only durable when in the acid a small siliconcontent was continuously maintained. The volatile siliceous substancesSiCL; and silicone oil reacted under the conditions of Tests 17 to 35with Concentration Tempera- Duration Linear corrosion the acid, wherebya nonvolatile siliFeol-ls Teaction Product Test of a 1n We e of test,velocltyy 20 was formed in the liquid. This reaction product guaranteednumber percent C. hours min/year a protection of the titanium in theliquid phase against 1 3 48 xg g gg g corrosion for a longer period oftime whereas the initial 2 200 43 Highe 'than intest protective effecton the titanium in the vaporous phase 3 30 200 144 2 ceased earlier.41:11:11: 00 200 48 3I5I 25 In Tests 18-35 about 20 milligrams ofphenylmethylg 22 2 22g g-gsilicone oil (density 1.02 g./cc., heatstability in air at 71111111 75 164 11s Iii glierthani te t 250 C. 1000hours, viscosity 200 centistokes at 25 C.) was added to about cc. ofacid medium. After the The values could not be exactly defined.respective duration of the tests the titanium exposed to TABLE2 Concen-Stability of titatration of Tenipera- Duration Amount of nium in IINOQin ture of of test, addition percent HNO; hours Addition in g. V L

30 200 144 Si (powder) 0. 2 30 200 144 Iron silieide 0.2 30 200 1443101161120.. 0.1 30 200 144 Quartz powder. 0. 2 30 200 48 Na silicate0.00 30 200 (la-silicate. 0. 02 30 200 0.2 30 200 0.1 30 200 0.15 30 2000.2 30 200 0. 02 3 200 0.02 10 200 0. 02 30 245 0.02 30 205 0. 02 00 2000. 02 00 205 0. 02 05 140 0. 02 75 164 0. 02

1 The volume of acid filled in was about 30 cc. 1 V=vaporous phase;L=liquid phase.

3 A phenyl-methyl silicone oil was used; density 1.02 g /co., viscosity200 centistokes at 25 0; thermal stability in air at 250 0. 1,000 hours.

TABLE 3 Percentage in acid Linear corrosion v1 Protecting efiect of 0.02g. silicone oil added to the 1 V=vapor0us phase; L=liquid phase.

+ means a thin layer wasiormed 011 the titanium; a loss in weight wasnot observed. to means the protective layer was not uniform, some spotsof the metal surface were corroded.

the attack of the vaporous phase of the acid was not yet corroded.Further tests showed that a permanent protection of the titanium in thevaporous phase could be achieved when a small amount of silicone oil wasadded in convenient intervals to the acid. A sample of titanium metalcovered with a thin film of silicone oil and placed into the titaniumtube likewise showed an effective protection against corrosion. Theweight of the silicone film was 5 milligrams. After a time of action of15 hours of a 60% HNO at 200 C. the titanium tube and the samples oftitanium in the liquid as well as in the vaporous phase did not shOW anycorrosion.

In all cases where an anticorrosive effect was obtained this effect wasextended to the gap due to construction on the bottom plate attached bywelding and on the sealing plate of the used test tube.

For the tests with titanium alloys corresponding metal samples wereplaced into the specified tubes of pure titanium. Tests were carried outwith the following alloys: titanium with 6% of aluminum and 4% ofvanadium, titanium with 5% of aluminum and 2% of molybdenum, andtitanium with 0.2% of palladium.

The results are summarized in Table 4. It is obvious that theanticorrosive eifect obtained with pure titanium is also reached withtitanium alloys.

What is claimed is:

1. A process for protecting titanium and titanium alloys, having as analloying component aluminum, vanadium, molybdenum or palladium with thebase metal being titanium, against corrosion by a strong oxidizing acidmedium at temperatures exceeding 100 C., which comprises allowing theoxidizing acid medium to act upon titanium or titanium alloys in thepresence of silicon and a siliceous substance selected from the groupconsisting of iron silicide, Si .xH O, quartz, sodium silicate, calciumsilicate, glass, kaolin, asbestos, SiCl halogenosilanes, and silconeoils.

2. A process as claimed in claim 1, wherein the oxidizing acid medium isallowed to act upon titanium or titanium alloys in the presence of SiCl3. A process as claimed in claim 1, wherein kaolin, asbestos, quartzsand, glass powder, precipitated silicic acid, sodium silicate orcalcium silicate is used as the siliceous substance.

TABLE 4 Protecting efiect of 0.02 g. Percentage of v (mmJyear) siliconeoil added to the in acid mixture Duration when the acid acid mixture 1of test at Titanium is free from Test number HCl HNOa 200 C. alloyadditions V L 0 30 48 P 2. 4 0 30 48 V 3.5 0 30 24 M 1.1 0 60 22 P 0 6022 V 0 60 22 M 10 24 P 10 22 P 3. 1 10 20 22 V 0.5 10 20 22 M 1. 3 10 22P 10 45 22 V 10 45 22 M 1 Composition of Ti alloys: P=Ti with 0.2% Pd;V=Ti with 6% Al. 4% V; M=Ti with 5% Al, 2% Mo.

2 See footnote 1 of Table 3. 3 Not determined.

References Cited UNITED STATES PATENTS 1,890,595 12/ 1932 Valenta.

2,661,286 12/1953 Swazy et al 175.5 2,678,875 5/1954 Spooner 252387 X2,711,364 6/1955 Beach 1343 X 2,711,974 6/1955 Happe 117-127 2,739,0473/ 1956 Sanz.

2,961,110 11/1960 Cooke et a1. 117135.1 X 2,981,610 4/1961 Snyder et a1.

3,010,854 11/1961 Satterfield 252-793 X 3,231,374 1/1966 Sciambi.

OTHER REFERENCES Bishop, Corrosion, vol. 19, 1963, pp. 308t, 313t;

TA462 C58, Sci. Lib.

Gordon et al., Surface Coatings and Finishes, 1954, p. 221, ChemicalPub. Co. Inc. New York, N.Y., TP936 RALPH S. KENDALL, Primary ExaminerU.S. Cl. X.R.

